Closing capacity

Closing capacity
= the lung volume at which the small airways in (usually the dependent part of) the lung first start to close
--> Impairs gas exchange and increase venous admixture
--> Decrease PaO2

NB:

• Closing capacity
  = RV + closing volume

Variation of closing capacity

Closing capacity increases with age

FRC does not change with age

• Young subject, closing capacity = 10% of VC
• In erect position, closing capacity = FRC = 40% of VC at 66 y.o.
• In supine position, closing capacity = FRC at 44 y.o.
• In neonates, lung elastic recoil is reduced
  --> More airway closure
  --> Closing capacity > FRC
  --> Reduced PaO2

Measurement of closing volume

Closing volume is measured with single breath nitrogen test

1. A single breath of 100% O2 from residual volume
2. Slow expiration with a rapid nitrogen analyzer
3. Late in expiration when airway closure starts to occur, expired [N2] starts to rise about the plateau.

Closing volume = the volume expired, from the start of this rise to the end of maximal expiration

Rationale

When inspiring from residual volume

Airway is closed in the base

1. Because initial part of the inspired gas (which is dead space gas, with high [N2]) goes preferentially to apex,
   AND,
2. Because apical alveoli were more expanded at the start and expands less during inspiration, [N2] is less diluted

--> Thus apical [N2] is higher

When basal airway starts to close at closing capacity

--> The portion of gas that is coming from apical alveoli increases

--> Expired [N2] starts to rise